1. Field of the Invention
This invention relates to metal detectors of the type used to detect metal fragments in foodstruffs and pharmaceuticals, and is directed more particularly to an improved search coil frame member, frame assembly and search coil assembly, and method for making same.
2. Description of the Prior Art
In many industries, non-metallic materials, such as foodstuffs and pharmaceuticals, are conveyed automatically to various processing machines and stations. A problem arises when tramp metal is introduced into the material stream. Such tramp metal can constitute a hazard if sold to the public in foodstuffs or pharmaceuticals, and even in the processing area can constitute a hazard for some types of processing machinery.
It is known in the art that non-metallic material can be passed through an electromagnetic field of a search coil and that if tramp metal is present in the material stream such metal will disturb the electromagnetic field of the search coil and induce a detectable error signal. Responsive to the error signal, the material stream can be stopped or diverted to a waste container, accompanied, if desired, by visual and audible alarm means.
It is known in the art to employ a conductive shield between the electrical conductors which generate the electrical field and the product under inspection for metal contamination. The shield may comprise a conductive coating, such as a dispersion of carbon emulsion, or a graphite. The shield acts to improve the discriminating abilities of the metal detection system when the products under inspection are themselves conductive to high frequency currents. Further, the shield operates to reduce spurious signal effects created by product transport mechanisms, such as belts and chutes.
In a method used in the manufacture of metal detector search coil assemblies, a correct size frame is made having an aperture of desired shape and size. The internal walls of the aperture are coated with a graphite or carbon emulsion to provide the above-described electrostatic shield and over the graphite or carbon there is applied a coating of plastic, as by spraying, dipping or cementing sheets of hard plastic to form a protective layer for the aperture. On the external walls, conductive strands are wound on the frame which is then attached to a rigid metal housing which provides an enclosed void around the exterior of the frame and strands. The space between the strands and the housing is then filled with non-metallic filler, such as an epoxy or other potting plastic. The filler acts as a moisture sealant, as well as a retainer for reducing the relative motion between the metal housing and the strands. Any relative motion between the strands and the surrounding metal housing results in a spurious signal.
The carbon or graphite coating is provided with a gap extending in the direction of travel of the product through the aperture. The conductive coating is electrically connected to a reference ground system in a manner normally used to provide a discharge path for static electricity.
The above-described apparatus is vulnerable to deterioration because of moisture penetration of the aperture plastic coating, aggravated by temperature cycling.
In U.S. Pat. No. 4,814,734, issued Mar. 21, 1989 in the name of James M. Moran, and U.S. Pat. No. 4,819,321, issued Apr. 11, 1989 in the name of James M. Moran, there is disclosed a metal detector search coil assembly, and an alternative method for making same. The assembly there disclosed includes, among other things, the provision of a non-conductive coil frame, the application of a layer of conductive carbon material to the external surfaces of the search coil assembly frame, and a layer of insulating plastic material applied over the carbon material. Vinyl insulating strips are placed on the plastic layer in positions where the coil is to be located so as to increase insulation between the conductive shield coating and the coil windings. A disadvantage of this construction is that the conductive coating is positioned quite close to the coil wires.
An object of the present invention is to provide an improved structure and method which facilitates positioning of the shield between the coil conductor wires and the product under inspection, and in which no additional insulation materials are required.
A further object of the invention is to provide a search coil structure and method providing an aperture in which the exposed surfaces are stable and rigid and able to withstand the use and environmental changes to which the aperture surface is subjected.
With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a metal detector search coil assembly comprising a frame assembly having an aperture therethrough, electrically conductive strands wound upon the frame assembly, potting plastic disposed on external surfaces of the frame assembly and embedding the strands, and a metal housing enclosing the plotting plastic, the frame comprising a plurality of frame members joined together to form the aperture, each of the frame members comprising a first sheet of rigid plastic, a second sheet of rigid plastic of substantially the same configuration and substantially the same size as the first sheet, an electrically conductive material disposed between the first and second rigid plastic sheets, and adhesive disposed on the sheets to bond the sheets together with the conductive material bonded therebetween.
A further feature of the present invention is a method of making the above-described metal detector search coil assembly.
The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular device and method embodying the invention are shown by the way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.